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ECE 445: SENIOR DESIGN PROJECT

Spring 2006 Wireless Remote Motor Controller By Joseph Owusu Sara Shahzad Masakatsu Suzuki TA: Scott Anderson Project No.

Monday, May 1, 2006

ABSTRACT

To enable us to control a motor from a distance in excess of 300ft, we decided to build a wireless remote motor controller. It encompasses a control circuit, receiving circuit and some power electronics circuit. The control circuit basically starts, stops, accelerates, and decelerates a motor. The control circuit is made of a potentiometer which varies analog input voltage to an A/D converter; the analog voltage is converted to digital voltage by the A/D converter and sent to a Decoder. The decoder decodes the signal and sends it to a transmitter. The receiving circuit receives the signal through the receiver and then a reverse engineering process is performed .The analog voltage received at the receiving circuit is used to control a pulse width modulation (PWM) signal, which in turn controls the speed of the motor through the power electronics circuit made of a buck converter and H-bridge. ii

• 1. INTRODUCTION....................................................................................................................
  • 1.1 Objectives...........................................................................................................................
  • 1.2 Benefits...............................................................................................................................
  • 1.3 Features...............................................................................................................................
  • 1.4 Performance Requirements……………………………………………………………….
  • 1.5 Block Description…………………………………………………………………………
• 2. DESIGN PROCEDURE...........................................................................................................
  • 2.1 Design Decisions…………………………………………………………………………. - 2.1.1 Power Supply Design…………………………………………………………….. - 2.1.2 User Input Circuit Design……………………………………………………….... - 2.1.3 Data Transmitter and Receiver Design…………………………………………… - 2.1.4 PWM generation Design……………………………………………………….…. - 2.1.5 H-Bridge…………………………………………………………………….……. - 2.1.6 Converter Design…………………………………………………………………. - 2.1.7 Switch Selection………………………………………………………….……….. - 2.1.8 Inductor Design…………………………………………………………………… - 2.1.9 Capacitor Design……………………………………………………….…………..
    • 2.2 Simulation.....................................................................................................................…...
• 3. DESIGN DETAILS............................................................................................................…
  • 3.1 Inputs…………………………………………………………………………………….
  • 3.2 A/D Converter…………………………………………………………………………...
  • 3.3 Encoder (HT-640)...……………………………………………………………………..
  • 3.4 Transmitter………………………………………………………………………………
  • 3.5 Receiver………………………………………………………………………………….
  • 3.6 Decoder(HT-648)………………………………………………………………………..
  • 3.7 D/A Converter…………………………………………………………………………...
  • 3.8 PWM………………………………………………....………………………………….
  • 3.9 H-Bridge……………………………………………………………...………………….
  • 3.10 Buck Converter…………………………………………………………………………
• 4. DESIGN VERIFICATION.....................................................................................................
  • 4.1 Testing Procedure............................................................................................................. - 4.1.1 RF Module……………………………………………………………………..… - 4.1.2 Buck Converter……………………………………………………………….…..
  • 4.2 Results............................................................................................................................... - 4.2.1 PWM generation…………………………...…………………………………….. - 4.2.2 Buck Converter………………..…………...…….……………………………….
• 5. COST......................................................................................................................................
  • 5.1 Parts..................................................................................................................................
  • 5.2 Labor.................................................................................................................................
• 6. CONCLUSIONS....................................................................................................................
• REFERENCES
• APPENDIX A. – SCHEMATICS..............................................................................….........A.
• APPENDIX B – TEST DATA……………………………………...……………………….A.
• APPENDIX C – PICTURES………………………………………………………………...A.

1. INTRODUCTION

We found this project idea and a couple of others on the ECE Power Webpage that immediately draw our interest. We decided to go with this project in specific because of its remote control capabilities, which we figured will be fun to do and also expose us to the RF field of electrical engineering, which none of us really have worked with before, and thought it would be a good learning opportunity. All of the team members have a power background and therefore, are pretty conversant with the motor controller part. Adding the remote control capability is going to be our biggest challenge, and will push us out of our comfort zone, thereby helping us to learn more---this is all ECE 445 is about anyways. We also found a couple of motor controllers on the market, but they all came without remote control and pretty expensive as well. So we figured that if we can successfully build this motor it could be very marketable at low-cost, and could be a good basis for a very efficient robot or maybe a wireless car. 1.1 Objectives The objective of this project is to design, build and test a wireless remote motor controller which will come in handy for applications such as in a golf cart. The user will be able to do the following operations: start, stop, accelerate and decelerate the motor, by pushing a button or switch from a distance. 1.2 Benefits  Inexpensive  Simple and efficient to use  Easy control 1.3 Features  Allows adjustable speed control  Motor will be able to run 150 W continuously and 250 W for at least one minute  Requires less power ( power from 12V lead acid battery)  Remote control capability 1.4 Performance Requirements  Input voltage of 12 Vdc  Output voltage ranging from 0 V to 12 Vdc, depending on the user specification.  Motor loads ranging up to continuous 150 W or 250 W for a minute.  Efficiency greater than 85%.  Current ripple ± 5%.  Voltage ripple less than ± 2%.  Wireless control in distances in excess of 300 ft.

Buck Converter: The buck converter, or a step-down converter, is a switching dc-dc converter that will produce output voltage lower than the source. It will have an input voltage of 12 V from the lead-acid battery and will be designed to output voltage from 0 V- 12 V. The output voltage will determine the duty ratio from the control circuit and this output voltage of the converter will be interfaced to the motor to control its speed. Motor: A 12 V permanent magnet dc motor will be supplied by a user. It will be used up to continuous load at 150 W and 250 W for one minute.

2. DESIGN PROCEDURE

2.1 Design Decisions 2.1.1 Power Supply Design The main power supply for this project will be a 12 V lead acid battery. It will supply the power to the converter circuit and the control circuit. Since many of the components required less than 12 V as their VCC, we needed to design a voltage regulator in order to convert from 12V to 5V. We used 78L05 as our voltage regulator, which we found at the ECE part shop. We also needed a voltage inverter to supply negative VCC. Since negative voltage regulator does not exist as a single chip, we researched and built a voltage inverter circuit. 2.1.2 User Input Circuit Design Sine PWM pulse generated depends on its PWM chip, we designed a circuit such that user can vary the voltage at certain range. We chose the input voltage range arbitrarily after realizing the voltage generated from the user controlled circuit will be converted to digital signal and it will be reconverted back to analog signal with desired reference voltage. We used two potentiometers to allow the user to vary the voltage. The first potentiometer is used to set the maximum allowed voltage, and the second potentiometer is used to vary the voltage from zero to the maximum. We used ADC0802 as our A/D converter. In the later stage of the testing of the project, we recognized a need to create a “kill” switch. By implementing this switch, we were able to set the duty ratio to zero immediately. Without this switch, the user had to manually decrease the duty ratio to zero. 2.1.3 Data Transmitter and Receiver Design We initially did not have any idea how to a wireless system worked, thus we looked into student resources section of the ECE 445 website for a help. We came across the page which discussed how to use the LINX modules. After researching about RF modules from the LINX website, we came to conclusion that the RF module available from students in the senior design project is sufficient in what we were trying to achieve. The RF module we obtained from the department is TXM-900 and RMX-

900. We also realized that the signal must be encoded and decoded for us to use the RF module effectively. Again, we researched about the encoder and decoder and explored couple ideas. We initially thought about designing an encoder and decoder set by programming microcontroller, but we decided to use HT640 and HT648. HT640 and HT648 features low power and high noise immunity CMOS technology, low standby current, an easy interface with RF with media, and minimal external components. Although it might have been possible to design our own encoder and decoder and reduce the cost, we decided to focus our time on other aspect of our project. 2.1.4 PWM generation Design One of the most important decisions we had to make was choosing the method of pulse-width- modulation. Because we were controlling the duty ratio wirelessly, the initial signals received from the remote controller were digital signals. We explored the idea of generating the PWM from both digital inputs directly and from analog inputs. Digital method would have involved a comparator which compares digital signals and a digital value generated from a counter of equal bits. The output will be “on” when the value of counter is greater than the value of digital signals and “off” when otherwise. An analog method would be achieved by converting the digital signal from the remote controller to an analog signal. After signal is converted to analog, a pulse wave can be constructed from the use of PWM

max max

P

I A

V

Since, the maximum continuous load was 150W, the converter was being designed for that rating. It was realized that the maximum power would result at the highest output voltage which was 12V in this case. Hence, substituting these two values in equation (1), the maximum output current was computed to be 12.5A. However, with this calculated output current, the input current would have to be higher in order to compensate for the inevitable power losses due to wires and the power dissipation that occurs through the various switching components. The power lost due to inductor and capacitor will be all reactive, therefore these two components would not affect the efficiency of the converter as much. In order to account for these losses, it was decided to design the converter instead at around 250 W of maximum load, which resulted in the rated current being 20.833A. Even though this rating was a high overshoot of the actual input rating, a safe overestimation was better than the potentially dangerous consequences of underestimating. L=257.1uH 220uF Vz 12V 0.96-28.8 ohms Figure 1. Buck Converter Configuration As seen from Figure 2, the buck converter incorporates a mosfet, a diode, a capacitor and an inductor. The appropriate choice of these components is very vital for the performance of the buck converter. Especially, because high efficiency was one of the goals of the project, it was desired to have minimum power lost through switches. Naturally, the next step now was to pick the switches and compute the capacitor and inductor values ideal for our design. 2.1.7 Switch Selection The fundamental concept of dc-dc conversion is the use of switching to change the voltage from the input to the output. The MOSFET acts as the first switch, which is controlled by the duty cycle supplied by the gate drive. When the duty cycle supplied by the PWM chip is high, the positive bias at the gate will cause the current to flow freely from the drain terminal to the source terminal of the MOSFET. Therefore, in a case where the duty ratio is high, the MOSFET can be viewed as a shirt circuit. However, when the duty ratio is low, the MOSFET acts as an open circuit preventing any current from flowing through between the drain and the source terminals. Although, when the MOSFET does act like an open circuit, there is a small voltage drop across it. Keeping in mind the high current ratings, initially IRFP360 was chosen as the MOSFET for the converter. This MOSFET was rated at VDSS= 400V and ID= 23A with a drain to source resistance of 0.2Ω. Because of little experience with the selection of switching devices, this resistance seemed low

enough for this design and was chosen for the converter. Soon, by doing the PSPICE Simulation (discussed later in the report) it was realized that this resistance was actually pretty high and kept the converter from reaching the desired efficiency. Hence, after looking through a lot of other MOSFETS that were available, IRL7833 seemed to be a better choice. This MOSFET has following ratings: VDSS= 30V, ID= 30A and RDS(on)= 3.8mΩ. The high current ratings and lower source-drain resistance made this MOSFET more appropriate for the design. Due to such high currents flowing through the MOSFET, it would tend to heat up sometimes, and hence heat sink (TO-247) was placed on it, to prevent the temperature from rising up too much. The diode that was chosen was MBR2545. This schottky diode had the ratings of IF(AV) = 3 0A, VR = 45V, and VF = 0. 73 V. This diode chip had a total of three legs as oppose to a simple two leg configuration. Two out of the three legs were anode, with one cathode leg. This allows the user to connect both the anode legs together, which would be the same as having two diodes in parallel. Hence, this would reduce the power dissipation and also the diode would not heat up as fast because of the current being split into two legs. A TO-247 heat sink was still placed on the diode for precautionary measures. The forward voltage drop was pretty low for this diode, despite of the high ratings. The current limit was satisfactory for this design as well, making MBR2545 suitable for the converter. 2.1.8 Inductor Design With the switches determined, the next step was to compute the values for the storage components. In order to find the inductance value for this design, the critical inductance had to be calculated first using equation (2). max

critical in [^ ]

V DT

L H

I

where D is the duty ratio and T is the time period. To compute the worst case critical inductance, the duty ratio is set to 1 in equation (2). However, since the switching frequency was chosen to be 50 kHz, the time period is 1 1 2 5sec 50, 000

T E

f

Now, plugging in all the values back in equation (2), the critical inductance turns out to be (12)(1)(2 5)

critical

E

L uH

The critical inductance is just a measure of the minimum inductance for the design. So, the actual inductance value had to be much greater than the critical value as long as the saturation (Bsat) is not reached. The goal now was to design an inductor such that the inductance is greater than the critical inductance and yet the core is not saturated. For this purpose powdered-iron core was used, around which the wire was to be wound in order to make a desirable inductor. In order to do so, the parameters of the core were determined. The length of the core turned out to be 0.198m and the area was 3.38E-4 m^2. Since two similar powdered iron cores were joined together to make the inductor, the effective area doubled. This was done because the larger area would tend to yield a lower reluctance, requiring the number of turns to decrease in order to result in an inductance value that works for the circuit. Since the window of the cores limited the number of turns that could be wound around it, a bigger reluctance was not desirable. In addition to this, a larger number of turns might tend to saturate the core, which was undesirable. Also, the permeability of the core was 75μo. Hence, the reluctance of the core was then computed using the above values to be:

replaced the MOSFET with IRL7833 which had a much lower drain to source resistance. Since, this new MOSFET was not available in the parts library of the PSPICE, we were unable to simulate the circuit again with the newer MOSFET. V 1 1 2 1 2 L 1 2 5 7. 1 u H V 2 T D = 0 T F = 0. 0 0 0 0 0 0 0 0 1 P W = 0. 0 0 0 0 2 P E R = 0. 0 0 0 0 2 V 1 = 0 T R = 0. 0 0 0 0 0 0 0 0 1 V 2 = 1 2 0

BUCK CONVERTER

M 1 I R F P 3 6 0 C 1 2 2 0 u F R 4

1. 0 2 5 R 5
2. 9 6 D 2 M B R 2 5 4 0 I I V V Figure 2. Buck converter schematic for PSPICE

3 DESIGN DETAIL

The schematics for each of the described components can be found in Appendix B. 3.1 Inputs This will be a 5k potentiometer (VR1) and 20k trimming potentiometer (VR2) and an op-amp. The potentiometer VR1 varies the voltage to the A/D converter Vin+ .The trimpot VR2 has to adjust so that the maximum input to Vin+ of A/D converter will not exceed 1.25 since we will set our reference voltage to 2.5V 3.2 A/D Converter The A/D converter will converter analog voltage to 6 bit parallel data starting from the least significant bit(LSB) based on the amount of varied voltage it receives from the input. The other 2 bit sends out signal to motor control. To do this CS and RD pin are tied to ground and connecting WR to INTR. This also allows the device to operate in the free running mode. 3.3 Encoder(HT-640) The data inputs of the encoder receive 6 data bits from parallel digital signal from the A/D converter. Two data bits are sent to the encoder with switches. Thus, 8 bits are sent to transmitter serially from Dout. 6 out of the 8 address bits are used to match the 6 address bits of the decoder. Other 2 are grounded. Oscillating frequency is set to 50 kHz by setting resistor to 680 K ohms 3.4 Transmitter The transmitter will receive the serial data from Dout of the encoder. A precision 12.00MHz Voltage- Controlled Crystal Oscillator (VCXO) serves as the frequency reference for the transmitter. Incoming signals are filtered to limit their bandwidth and then used to directly modulate this reference. The modulated 12.00MHz reference frequency is applied to the Phase-Locked Loop (PLL). The PLL, combined with a 902-928MHz VCO, forms a stable frequency synthesizer that can be programmed to oscillate (An on-board micro-controller manages the PLL programming functions and greatly simplifies user interface) at the desired transmit frequency. 3.5 Receiver The RF signal coming in from the antenna is filtered by a Surface Acoustic Wave (SAW) filter to attenuate unwanted RF energy (i.e., not in the 902-928MHz band). Once filtered, the signal is amplified by a Low-Noise Amplifier (LNA) to increase the receiver sensitivity and lower the overall noise figure of the receiver. After the LNA, the signal is mixed with a synthesized local oscillator operating 34.7MHz below the incoming transmission frequency to produce the first Intermediate Frequency (IF). The signal is routed to the analog output pin and the data slicer stage, which provides squared digital output via the data output pin 3.6 Decoder (HT-648) The data input of the decoder receives serial data from the receiver. Data output of the decoder send 6 out of 8 data bits to the D/A converter. The other 2 data bits are sent to H-bridge inputs for direction and enable control. 6 out of the 8 address bits matches the 6 address bits of the encoder. Other 2 are

4. DESIGN VERIFICATION

Due to the high power aspect of this project, the laboratory equipment often created hurdles for us to overcome. 4.1 Testing procedure 4.1.1 RF Module Before we tested the RF capability, we tested to see if the transmitter and receiver work. We set up a function generator to make a 1 kHz TTL compatible square wave. We connected this signal to the data input lead from the module. We verified that it is functional by connecting the data output of the reference receiver into an oscilloscope. At the oscilloscope we were able to observe the identical waveform. When we tested the module with a sine wave, we were not able to observe the waveform at the oscilloscope. We later learned that sine wave is difficult to transmit since RF module was designed to transmit digital signals (pulses). 4.1.2 Buck Converter In order to test the performance of the buck converter, various data was acquired with various loads. Since, the motor for this project was to be supplied by the user, and the main goal here was just to make an effective and economical wireless remote motor controller, we did not really have a motor that was rated according to the purpose of this project. Hence, we used resistors with different values as our varying load, from 0-150W. With different loads, we measured the input and output power, the efficiency of the converter, the temperature across the gate drive MOSFET and the voltage ripple. This data can be found in table 1 of the appendix. 4.2 Results 4.2.1 PWM generation Before we interfaced the buck converter with the rest of the circuit, we tested the PWM control circuit using wireless remote control to make sure it can generate the desired pulse waveform. We captured the pulse waveform at the output of the PWM control circuit by connecting it to the oscilloscope. We were able to generate the desired pulse waves. Figures 4 and 5 show the output pulse wave at duty ratio of 96.08% and 58.37% respectively.

Figure 3. Pulse wave at 96.08% duty ratio Figure 4. Pulse wave at 58.37% 4.2.2 Buck Converter As for the results obtained for the converter testing, the voltage ripple stayed pretty much within 2% for the most part, as was our goal. However, at lower resistances and higher output power, it rose up to about ±4%. This could have been improved by using a higher capacitance, so that the ripple would always stay below ±2%. The data for each of the different cases that were tested can be found in table 1 of the appendix. Figure 6 shows the voltage ripple plot that was obtained on the oscilloscope. Figure 5. Voltage ripple plot for the buck converter The next important performance aspect of the converter which was analyzed was the efficiency. The efficiency was pretty high for duty ratio between 50-85%. However, for low duty ratios and very high duty ratios, the efficiency was low. This was because there was always a fixed amount of power lost due

Effic ienc y versus Duty Rat io 0 10 20 30 40 50 60 70 80 90 100 20 35 50 65 80 95 Duty Rati o (%) R=1.8 ohms Figure 7.5. Resistive load of 1.8Ω, with power load of 80W It was important to monitor the temperature of the gate drive MOSFET, to ensure that nothing gets burned or heat up too fast. Hence, we took the temperature data which is shown in Figure 8. This plot verifies that the temperature was under control and hence all the connections were right, thus nothing was burning. Figure 8. The Temperature of the Gate drive MOSFET at different output loads

5. COST

5.1 Parts The description and the cost of the parts we used for this project is shown in Table1. The total cost of the project components is calculated as $164.45. This cost could be reduced if we buy some of the parts in larger quantities. TABLE 1. PARTS DESCRIPTION AND COST Part # Mft Desc Price Qt y Total KA78L05A FAIRCHILD Voltage Regulate $.590 1 $. 3296W-1-203 BOURN TRIM POT $2.20 1 $2. CU5021 OHMITE Potentiometer $2.20 1 $2. TXM-900-HP3 LINX Transmitter $30.33 1 $30. RXM-900-HP3 LINX Reciever $46.03 1 $46. LM358N FAIRCHILD OP-AMP $.26 1 $. ADC0802LCN NATIONAL A/D Converter $6.24 1 $6. TL072CP MOTOROLA OP-AMP $.64 1 $. DAC0808LCN NATIONAL D/A Converter $1.74 1 $1. 1N4004 MOTOROLA Diode $0.10 1 $0. TL598 T I PWM Chip $1.35 1 $1. IRFP360 IR MOSFET $4.061 5 $20. T300-26 DEXTER Core $5.00 1 $5. ANT-916-CW LINX? Antenna $7.04 2 $14. RG-174 50W MCM Coaxial cable $2.44 1 $2. 76SB08 GrayHill switches $1.49 4 $5. SN7416N TI inverter $0.67 1 $0. MC4427 FREESCALE Buffer $.50 1 $0. HT-648L HOLTEK Decoder $3.42 1 $3. HT-640L HOLTEK Encoder $3.42 1 $3. MC33883DW FREESCALE H-Bridge Driver $3.63 1 $3. P5310-ND PANASONIC 22.5u Capacitor $0.39 1 $0. VR9-3134-000 1.5K Resistor $0.35 3 $1. P10.0KFCT-ND PANASONIC 10K Resistor $1.78 2 $3. 97-20K MCELECTR ONICS 20 K Resistor $1.83 1 $1. VV1-2115-684 CHIP RESISTOR 680K Resistor $0.16 2 $0. C001U250M (^) MYLAR 0.001u Cap $0.10 1 $0. GT1213 GASTON Battery $5.50 1 $5. 3352W-104-ND Bourns 1K potentiometer $0.60 1 $0. Total $164. 5.2 LABOR We anticipated our future salary to be $50 and therefore calculated the total cost involved in the project using equation: